# What is Einstein’s General Theory of Relativity?

The general theory of relativity is the mainthe building block of modern physics. It explains gravity, based on the ability of space to “bend”, or, more precisely, connects gravity with the changing geometry of space-time. Albert Einstein founded the “General” Theory of Relativity (GR) in 1915, ten years after the creation of the “special” theory, applying the universal speed of light and assuming that the laws of physics remain unchanged in any given reference frame. But is GR so complicated as it might seem at first glance?

The general theory of relativity is a geometric theory of gravitation, developing a special theory of relativity

How to understand the general theory of relativity?

Einstein's general theory of relativity can be expressed in just 12 words:“Space-time tells matter how to move; matter tells spacetime how to bend ”. But this is a short description made by a physicistJohn Wheeler, hides a more complex and deeper truth. In addition to quantum theory, the general theory of relativity is one of the two pillars of modern physics - our working theory of gravity and a very large theory of planets, galaxies and the Universe as a whole. It is a continuation of Einstein's special theory of relativity - but so massive that it took him 10 years, from 1905 to 1915, to move from one to another.

According to New Scientist, according to a specialtheory of relativity (STR) motion bends space and time. Einstein's GRT combined it with the principle noted by Galileo more than three centuries ago: falling objects are accelerated at the same speed regardless of their mass.

The pen and hammer that fell from the falling Leaning Tower of Pisa will hit the ground at the same time if you do not take into account air resistance.

Following Galileo, Isaac Newton showed that thiscan be true only if there is a strange coincidence: the inertial mass, which quantifies the body’s resistance to acceleration, should always be equal to the gravitational mass, which quantitatively determines the body’s response to gravity. There is no obvious reason why this should be so, but no experiment has ever shared these two values.

Just like he used constantthe speed of light to build a special theory of relativity, Einstein declared this a principle of nature: the principle of equivalence. Armed with this and the new concept of space and time as an intertwined “space-time”, you can build a picture in which gravity is only a form of acceleration.

Massive objects bend space-time around themselves, causing objects to accelerate towards them.

Although gravity dominates large spaceon the scale and near very large masses, such as planets or stars, it is in fact the weakest of the four known forces of nature - and the only, unexplained quantum theory. Quantum theory and general relativity are applied at different scales. This makes it difficult to understand what happened in the earliest moments of the Big Bang, for example, when the Universe was very small and the force of gravity is huge. In another situation, when these forces collide at the event horizon of a black hole, insoluble paradoxes arise.

For example, quantum mechanics has ways to take into account concepts like infinitybut if we try to do the same with the general theory of relativity, mathematics generates predictions that do not make sense.

Some physicists have hoped thatone day, a “theory of everything” will be able to combine quantum theory and general relativity, although attempts such as string theory and the theory of loop quantum gravity have so far yielded no results. Meanwhile, Einstein's general theory of relativity predicted that very dense clusters of mass could distort space-time so much that even light could not escape from it. Now we call these objects “black holes”, we can photograph the “event horizon” that surrounds these space monsters, and we are practically convinced that a supermassive black hole rotates in the center of each massive galaxy.

Mathematical equations of the general theoryEinstein's relativity, tested over and over, is currently the most accurate way to predict gravitational interactions, replacing those developed by Isaac Newton a few centuries before.

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But perhaps the greatest triumph of general theoryRelativity came in 2015, when gravitational waves were discovered - ripples in space-time caused by the movement of very massive objects. The signal that the two black holes connected and merged into one was a triumph of the painstaking, patient work done by the international team of LIGO VIRGO laboratory researchers. Read more about how experts are looking for gravitational waves today, read in the fascinating material of Ilya Hel. One way or another, the development of a quantum physical “version” of the general theory of relativity remains a constant goal of modern physics.